This invention relates generally to nickel-base alloy compositions and more specifically to a family of nickel-base alloys containing about 18 to 25 atom percent molybdenum in combination with low but critical amounts of certain other substitutional alloying elements which provide thermal stability to the metallurgical structure.
Early in the twentieth century, it was noticed that the addition of substantial amounts (above 15 percent) of molybdenum to nickel markedly improved nickel""s resistance to corrosion by reducing acids such as acetic, hydrochloric or phosphoric acids. However, with increasing amounts of molybdenum, the alloys became much more difficult, if not impossible, to work into common shapes. Therefore, the first commercially available alloy of this type, called simply alloy xe2x80x9cBxe2x80x9d, contained about 18 or 19 percent molybdenum (all concentrations herein are expressed in atomic percentages) along with significant amounts (7 to 12 percent) of iron (primarily from the use of ferro-molybdenum in the manufacturing process, but also often added to reduce cost) as well as several percents of incidental additions or impurities including carbon, manganese and silicon. See, for example, U.S. Pat. No. 1,710,445 granted in 1929 to a predecessor of the present assignee.
While these alloys were relatively easy to cast into shapes, great difficulty was encountered in hot working them into plates and sheets for later fabrication into chemical vessels, piping and the like. During the 1940""s, the developer of alloy B, Haynes Stellite Co., continued to work toward improving this alloy family and, among other things, determined that copper was one of the elements most detrimental to hot workability. As disclosed in U.S. Pat. No. 2,315,497, the corrosion rate was unaffected by keeping the copper content below about 0.15 percent. Therefore, even today, copper is maintained as low as possible and preferably below about 0.5 percent.
Such alloys had good resistance to wet corrosion by non-oxidizing acids so long as the formation of second phase precipitates was avoided. Such precipitates, usually forming along grain boundaries in the heat affected zones during welding, promoted rapid intergranular corrosion by depleting adjacent areas in molybdenum. Thus, all welded structures needed a solutionizing or stabilizing heat treatment (e.g., 1100xc2x0 C. for one hour) followed by rapid cooling to suppress such corrosion. This effect is discussed in more detail in U.S. Pat. Nos. 2,237,872 and 2,959,480.
Since such heat treatment is expensive and even impossible for large welded structures, many attempts have been made to improve upon the basic xe2x80x9cBxe2x80x9d alloy to stabilize or even avoid such harmful precipitates.
During the 1950""s, an extensive study was undertaken in England by G. N. Flint who, as reported in several publications and patents (see GB Patent No. 810,089 and U.S. Pat. No. 2,959,480), found that the harmful precipitates were carbides of the M6C type (either Ni3Mo3C or Ni2Mo4C) which were dissolved by exposure to temperatures above 1200xc2x0 C. during welding, then subsequently re-precipitated at grain boundaries during cooling.
Flint concluded that, while it is not practical to lower the carbon content enough to prevent all carbides, it is beneficial to lower the iron and silicon levels to increase its solubility somewhat. More importantly, he also thought that the excess carbon could be stabilized by the addition of several percent of vanadium and/or niobium which would form stable MC-type carbides that would be more resistant than M6C to dissolution and subsequent re-precipitation at the grain boundaries after welding. Thus, such a material was thought to be substantially free from intergranular corrosion in the softened-and-welded condition. However, it was noticed that corrosion could be induced adjacent the weld by a xe2x80x9csensitizingxe2x80x9d heat treatment at 650xc2x0 C. This fact was unappreciated until later.
A commercial version of the Flint alloy was introduced during the mid-1960""s as HASTELLOY(copyright) alloy B-282, but soon was withdrawn from the market when it was shown to suffer not only severe intergranular corrosion, but also higher general corrosion rates than the old alloy B. It is generally believed that the difference in performance between Flint""s laboratory samples and commercial wrought structures was due to the much higher levels of impurities in the commercial alloys (notably silicon and manganese) in combination with the longer times at higher temperatures required by the normal manufacturing process.
At about this same time, Otto Junker, in Germany, adapted Flint""s findings about carbide control to cast alloys which had very low levels of carbon, silicon, iron or other impurities (e.g., manganese) and without vanadium (see GB Patent No. 869, 753). Wrought versions of this alloy were developed by the assignee of the present invention and sold under the name HASTELLOY alloy B-2, in place of the withdrawn alloy B-282.
During the last 30 years, most attempts to improve the performance of alloy B-2 have involved reducing the total level of impurities introduced during the melting process. (Although a few inventors have tried to add a magic element, no such alloys have been commercially acceptable. See, for example, U.S. Pat. No. 3,649,255 which adds B and Zr). Today""s alloy B-2 is generally resistant to intergranular corrosion caused by carbide precipitation, but still may require an annealing heat treatment after certain other manufacturing operations.
It is now known that even relatively clean Nixe2x80x94Mo alloys can develop complex second phases after exposure to temperatures in the range of 600-800xc2x0 C. Such phases are not compounds containing other elements (like the carbide precipitates) but, rather, different crystalline microstructures, such as the ordered intermetallic phases Ni2Mo, Ni3Mo, and Ni4Mo. Such phases are very brittle and provide for easy crack propagation along grain boundaries. Further, such phases cause the adjacent matrix to become depleted of molybdenum and thus have a lower corrosion resistance than the distant disordered fcc matrix, which explains the xe2x80x9csensitizationxe2x80x9d noticed by Flint after his heat-treatment of alloy B at 650xc2x0 C.
While some increase in corrosion rates can be tolerated in most applications, the severe age embrittlement due to the ordering reaction often results in catastrophic failures in stressed structures (such as cold worked or welded vessels) exposed to these temperatures for even a short time. The kinetics of the ordering reaction in alloy B-2 are very rapid, compared to the ordering in lower molybdenum alloys. For example, U.S. Pat. No. 4,818,486 discloses a Nixe2x80x94Moxe2x80x94Cr alloy with about 17 atom percent molybdenum, which is said to have xe2x80x9cexcellent ordering characteristics after an aging time of only 24 hours.xe2x80x9d
It should be apparent from the foregoing that there has been a long-felt need in the art for a high molybdenum, nickel-base alloy which does not exhibit rapid, order induced, grain boundary embrittlement and, preferably, with no sacrifice in corrosion resistance.
The aim of the present invention is to overcome the disadvantages of the prior art as well as offer certain other advantages by providing a novel family of high molybdenum, nickel-base alloys having the general formula NiaMobXcYdZe where:
X is one or more (preferably two or more) required substitutional alloying elements selected from Groups VI, VII or VIII of the Periodic Table;
Y is one or more undesirable but permissible other metallic substitutional alloying elements;
Z is any nonmetallic interstitial elements present;
a is the atom percent nickel and is more than about 73 but less than about 77 atom percent;
b is the atom percent molybdenum and is between about 18 and 23 atom percent;
c and d are the atom percents of the required and permissible substitutional alloying elements X and Y, respectively, where the total c is a least about two percent and c plus d is between about 2.5 and 7.5 atom percent, provided no one element X is present in amounts greater than about five atom percent and no one element Y is present in amounts greater than about one atom percent; and
e is the atom percent of any interstitial element Z which may be present, and is as low as practical, but is tolerated up to a total amount of no more than about 0.2 atom percent.
This family of alloys is characterized by exhibiting greatly enhanced thermal stability, as well as superior corrosion resistance, as compared to the prior commercial alloy B-2.
Accordingly, the present invention also includes a process or method for increasing the thermal stability of high molybdenum, nickel-base alloys. This method includes, along with the usual steps manufacturing these alloys, the steps of determining the chemical composition of said alloy during the primary melting stage, determining the total amount of substitutional alloying elements present in the alloy at this stage, then, if necessary, adding additional alloying materials containing elements selected from Groups VI, VII or VIII of the Periodic Table in order to adjust the final composition to contain about: 73 to 77 atom percent nickel, 18 to 23 atom percent molybdenum, 2.5 to 7.5 atom percent in total of at least one but preferably two or more substitutional alloying elements, but no more than five percent of any one element, and any incidental impurities not significantly affecting the properties of the alloy.
Further, the total amount of substitutional alloying elements (SAE) present is preferably related to the total amount of molybdenum present by the equation: SAE plus 0.7 times molybdenum is between about 18 and 20. Therefore, to determine more closely the preferred amount of additional alloying materials to add during manufacturing, the equation may be rewritten as: SAE should be about 19 minus 0.7 times molybdenum concentration.
While the inventor does not wish to be held to any particular scientific theory, since the exact mechanisms are not clearly understood at this time, it is believed that the increase in thermal stability (as evidenced by the reduced rate of hardening at 700xc2x0 C.), provided to these alloys by adding a low but carefully controlled amount of substitutional alloying element X, is due to the more stable electronic configuration of the intermediate transformation phases which seem to slow the ordering kinetics by favoring the formation of metastable Ni2(Mo, X) rather than Ni3(Mo, X) or Ni4Mo within the metallurgical crystal structure. Of course, even metastable Ni2Mo should eventually degenerate into other phases, such as Ni4Mo, but any delay is usually beneficial for fabricators of the alloy.